Antarafacial processes

Predict the product of the following pericyclic reaction. Is this [5,5) shift a suprafacial or an antarafacial process ... 

Dihydrothiophene-1,1-dioxides (42) and 2,17-dihydrothiepin-1,1-dioxides (43) undergo analogous 1,4 and 1,6 eliminations, respectively (see also 17-38). These are concerted reactions and, as predicted by the orbital-symmetry rules (p. 1067), the former is a suprafacial process and the latter an antarafacial process. The rules also predict that elimination of SO2 from episulfones cannot take place by a concerted mechanism (except antarafacially, which is unlikely for such a small ring), and the evidence shows that this reaction occurs by a non-concerted pathway.The eliminations of SO2 from 42 and 43 are examples of cheletropic reactions, which are defined as reactions in which two a bonds that terminate at a single atom (in this case the sulfur atom) are made or broken in concert. 

The above selection rules, therefore, predict that [1, 5] hydrogen shifts in neutral polyenes would be thermally allowed and the reaction would be facile, but thermal [1, 3] and [1, 7] shifts must go by an antarafacial process and they will be difficult to attain because of the geometric strain. This is also confirmed by many experimental observations. Thus concerted uncatalysed [1, 3] hydrogen shifts have not been seen in the diene of the following type, [1, 5] shifts are well known. 

There is evidence that a cydopentyne (generated in situ) also adds to a double bond by an antarafacial process Gilbert Bazc J. Am. Chem. Soc. 1984, 106, 1885. 

A suprafacial process is one in which the bonds made or broken lie on the same face of the system undergoing reaction. When the newly formed or broken bonds lie on the opposite faces of the reacting systems, it is known as an antarafacial process. 

Diels-Alder reactions of Ceo are generally believed to proceed via a thermally allowed concerted (suprafacial) process or a photochemical concerted (antarafacial) process [283-286]. However, an alternative stepwise (open-shell) mechanism for the Diels-Alder reaction has recently merited increasing attention [287-294], Along this line several reports describe an electron transfer with the formation of radical ion pairs as primary step of the Diels-Alder reactions, followed by a stepwise bond formation [295-301], The photochemical Diels Alder reaction of Ceo with an-... 

Overlap of lobes with like signs can occur by suprafacial-suprafacial addition or by an antarafacial-antarafacial process. The latter would not be expected for a [4 + 2] reaction because of geometric constraints. 

The selection rules predict an antarafacial process. In this case, suprafacial and antarafacial processes would lead to the same product. 

The [1,5] shift would be a suprafacial reaction. The [1,7] shift must be antarafacial in the v component to be thermally allowed. The proton, located at carbon A in 6-20, moves from the bottom side of the six-mem-bered ring to the top side of the w system at the other end. In general, the antarafacial process, necessary for a thermal [1,7] sigmatropic shift, occurs with ease, especially when compared to the antarafacial process that would be required for a thermal [1,3] sigmatropic shift. 

LUMO of ethylene. The dashed lines represent an antarafacial process on this component. 

Moving on to reactions in which antarafacial processes occur, we find that if we incorporate one antarafacial process, and keep the other suprafacial, then those reactions in which a total of 4n electrons are involved become allowed, and those in which a total of (4n + 2) electrons are involved become forbidden. To take just one of the relatively small number of reactions in this class, we can consider the remarkable reaction of heptafulvalene (129) with tetracyano-ethylene (130).128 This reaction is a [14 + 2] cycloaddition in other words,... 

Sigmatropic rearrangements involving a total of 4n electrons are much less common—one of the components must undergo an antarafacial process, and the geometrical requirements for this are often, though not always, severe. Here are some examples.143-145... 

However, there are alternative processes, called antarafacial processes, in which bonds are formed or broken on opposite faces of the reagent molecule. Let us consider again the addition of two ethylene molecules to form cyclobutane but with a different geometrical approach ... 

Fig. 13. Correlation diagram for the antarafacial process ethylene + ethylene - ...

The antarafacial process here can be excluded since it would produce a highly strained molecule (Fig. 16). In such rearrangements for bicyclo-heptenes and bicyclooctenes there are other examples, however, where the reaction is not so highly stereospecific, or even where retention is favoured over inversion. In such cases one might invoke a nonconcerted... 

Fig. 16. The product of the antarafacial process for [l,3]sigmatropic migration in bicyclo[4.2.0]oct-2-en-7-one...

Five concerted transition states are possible for Claisen and Cope rearrangements chair, boat, twist, cross and plane (Table i)274- 276, Since twist, cross and plane transition states are antarafacial-antarafacial processes that require highly elevated temperatures, only chair and boat transition states must be considered 54,277,278. 

Similarly, the rearrangement of the corresponding alcohol with anti configuration affords a 1 19 mixture of 12 and 13. Since both 12 and 13 have E configuration, the minor products must come from a net antarafacial process, probably by a nonconccrted, heterolytic mechanism384. This procedure has been used to establish the stereochemistry of the molecular fossil 13,16-dimethyloctacosane-l,28-diol. 

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